Electrochemical behaviour of low carbon steel in concentrated carbonate chloride brines

The polarization curves of low carbon steel in deaerated carbonate bicarbonate buffers (pH 10.3) show three distinct regions: (a) an active region characterized by a Tafel slope of 125 +/- 5 mV which is independent of the chloride concentration, (b) a gradual active to passive transition, and (c) a broad passive region. The addition of NaCl has a stronger effect on the passive than on the active region. For NaCl concentrations of 0.1 to 2 M, NaCl has only a small promoting effect (a reaction order of 0.1) on the anodic dissolution of the steel in the active region. Measurements of electrochemical impedance under free corrosion conditions confirm the above findings. The results are explained in terms of the more favourable formation of an iron hydroxy-carbonate complex (called green rust carbonate) than the formation of the corresponding green rust chloride complex. Passivity is attributed to the formation of a protective film containing both FeOOH and Fe2O3. It deteriorates with increasing chloride concentration and potential and improves with increase of buffer concentration. Evidence is also presented for metastable pitting, particularly in the presence of the lower chloride concentration, for example, 0.1, 0.2 and 0.5 M NaCl.